Experiments on driftwood-induced increase of backwater and hydrodynamic load at bridge deck

This study presents laboratory experiments with driftwood accumulation at a rectangular bridge deck placed in a flume with rectangular, smooth cross section. Driftwood (in the shape of isolated sticks with different length) was supplied in prescribed rate. Suitable placement of cameras onto the facility and appropriate image-processing methods returned the temporal evolution of the rate of incoming and leaving driftwood, the accumulation volume and projected area, and the backwater rise. Furthermore, the deck was not anchored to the channel walls but hanging from a trolley that could slide, its downstream motion being impeded by a force sensor that thus returned the hydrodynamic load acting on the bridge. For a Froude number of 0.27, characterizing the present experiments, it was experimentally verified that (i) the driftwood accumulation volume increases in time as new material is progressively added, even if part of the supplied material passes below the deck and proceeds to the downstream; (ii) most of the trapped material remains upstream of the bridge, while downstream the accumulation is limited to the material trapped within a flow recirculation area; (iii) the shape of the accumulation tends to achieve some self-similarity; and (iv) the backwater rise and the hydrodynamic load acting on the bridge are increasing functions of the accumulation size, and can double compared to the values for the bridge without any driftwood.